1. Field of the Invention
This invention relates to a catalyst system and a process for using same, useful for the stereospecific polymerization of butadiene. More specifically, the catalyst comprises an organoaluminum compound, a Lewis base, and a Ti(+4) halogen - M.sub.n I.sub.m composition. This catalyst system is useful in the production of polybutadiene homopolymers or copolymers in which the ratio of trans-1,4 to cis-1,4 units can be varied over wide limits and especially within the particularly desirable trans-1,4 range of 50 to 90% of total unsaturation.
2. Description of the Prior Art
Ziegler-type multi-component catalyst systems such as transition metal halides combined with organo metallic compounds have been known for well over a decade. The ability of such catalysts to polymerize butadiene to elastomeric products has also been recognized, but the polymers thus produced have generally been characterized by high cis-1,4 content rather than trans-1,4 addition units. Thus, for example, in U.S. Pat. No. 3,471,461, there is disclosed the use of TiX.sub.3.nAlI.sub.3 -xAl(alkyl).sub.3 catalyst systems for the polymerization of butadiene, in which the steric arrangement of the polybutadiene is predominantly that resulting from cis-1,4 addition.
Other catalysts, particularly based on vanadium compounds, capable of producing polybutadienes containing more than 90% trans-1,4 addition units have also been developed, but the polymers thus produced have been characterized by resinous or plastic rather than elastomeric properties.
We had previously discovered that certain trivalent titanium halide catalyst systems, used in conjunction with particular bases, produced polybutadienes containing a predominant amount of 1,4-addition units. These catalyst systems are taught in U.S. pat. No. 3,663,450 and U.S. Pat. No. 3,779,944.
In U.S. Pat. No. 3,663,450, a partially reduced titanium halide, notably TiCl.sub.3, is precomplexed with a Lewis base and forms a final catalyst capable of producing high cis-1,4 polybutadiene, while in U.S. Pat. No. 3,779,944 the Lewis base is not precomplexed with the titanium halide component and may be added to the polymerization zone in any order with respect to the latter component. This difference is important in that it provides for the production of polybutadienes having variable amounts of trans-1,4 addition units, up to about 90% in the polymer.
It is also known that certain titanium tetrahalides are useful in polymerizing butadiene. Generally, they produce polybutadienes containing 85-95% cis-1,4 addition units. Typical of these are a series of iodine-containing catalyst systems, such as TiI.sub.4 -AlR.sub.3, TiX.sub.4 -I.sub.2 -AlR.sub.3, and TiX.sub.4 -AlI.sub.3 -AlR.sub.3, where X is chlorine or bromine. These systems have been published in several patents. See Belgian Patent 551,851; Belgian Patent 602,496; U.S. Pat. No. 3,245,976, and British Patent 1,138,840.
Previously, when Lewis bases were added to titanium tetrahalide-type catalysts in amounts necessary to effect a noticeable change in the stereochemistry of the polymerization reaction, there was always an accompanying drastic decrease in the polymerization rate and catalyst efficiency.
The inventors have now found surprisingly that the addition of certain Lewis bases to a general catalyst system comprised of an organoaluminum, having the formula AlR.sub.a X.sub.3-a, and a Ti(+4) halide - M.sub.n I.sub.m composition causes a decrease in the cis-1,4 addition of the butadiene monomer and a corresponding increase in the trans-1,4 addition without the catalyst efficiency being adversely affected to any significant extent.
The inventors have also found that the novel catalyst system is capable not only of efficiently producing polybutadienes containing larger proportions of trans-1,4 addition units than those titanium based catalysts disclosed in the prior art, but also of producing predominantly elastomeric polybutadienes containing 50-90% of such units. This is a particularly surprising and significant discovery. Such polymers of corresponding over-all compositions which have been prepared in the past with the help of other catalysts or polymerization methods have generally exhibited a considerable amount of crystallinity and plastic character. While the inventors do not wish to be bound by any particular explanation for the difference in physical properties between the 50-90% trans-1,4 polybutadienes prepared according to this invention and those of the prior art, it is believed that the much more elastomeric character of the former is the consequence of the more random distribution of the cis-1,4 and trans-1,4 units in the polymer molecules than in the polymers heretofore known.
The purpose of this invention therefore is to provide a new catalyst for controlling the steric configuration of the 1,4 addition units of polymers of conjugated dienes, and copolymers thereof, and a process for employing same to produce such polymers and in particular to a polybutadiene containing a high percentage of trans-1,4 addition units and having predominantly elastomeric properties.